1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor sheet carrying a radiation image of an object stored thereon to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light to obtain an image signal for use in reproduction of a visible radiation image. This invention particularly relates to a radiation image read-out apparatus which prevents the contrast of the reproduced visible image from decreasing due to scattered radiation.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264 and 4,346,295 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored thereon, and is then scanned with stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet when it is exposed to stimulating rays is photoelectrically detected and converted into an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
However, when an object is exposed to a radiation for recording a radiation image in the aforesaid radiation image recording and reproducing system, radiation scattering (Compton scattering and Thomson scattering) is caused by elastic collision and electromagnetic interaction between the radiation and the object substance. The scattered radiation thus generated advances three-dimensionally in random directions, and impinges also upon the stimulable phosphor sheet. When the stimulable phosphor sheet is exposed to the scattered radiation besides the main transmitted radiation which carries the radiation image of the object and to which the stimulable phosphor sheet should be exposed for image recording, the contrast of the radiation image recorded by the main transmitted radiation on the stimulable phosphor sheet becomes low.
FIG. 2A is a schematic view showing the condition of radiation image recording on the stimulable phosphor sheet. Specifically, as shown in FIG. 2A, a stimulable phosphor sheet 10 is exposed to a radiation 6 emitted by a radiation source 5 constituted by an X-ray tube or the like and passing through an object 7 to have a radiation image of the object 7 stored on the sheet 10. At this time, a portion 8 exhibiting high radiation absorptivity and present inside of the object 7 is recorded as a "shadow" portion on the stimulable phosphor sheet 10. Thus the read-out image signal detected at one scanning line on the stimulable phosphor sheet 10 and representing the level of the radiation energy stored on the sheet 10 becomes as indicated by "a" in FIG. 2B, and a portion "c" at which the level of the signal "a" is markedly low corresponds to the object portion 8 exhibiting high radiation absorptivity. However, the read-out image signal "a" also carries the energy of a scattered radiation 6b in addition to the main transmitted radiation 6a shown in FIG. 2A. In the case where image recording is conducted on the stimulable phosphor sheet 10 so that the scattered radiation 6b is not generated, the read-out image signal detected from the stimulable phosphor sheet 10 becomes as indicated by "b" in FIG. 2B. Since, in general, the distribution of the scattered radiation 6b is approximately uniform as described later, it is assumed herein that the distribution is completely uniform. Namely, the read-out image signal "a" is in the form wherein a signal component Ss representing the energy level of the scattered radiation 6b is superposed on a signal component Sp representing the energy level of the main transmitted radiation 6a. However, since the read-out gain of a photodetector is usually controlled so that the background portion of the radiation image becomes of a predetermined level, the signal level difference is compressed and the contrast becomes low in the read-out image signal "a" as compared with the read-out image signal "b" as shown in FIG. 2C.
Accordingly, various attempts have heretofore been made to eliminate the adverse effect of the scattered radiation. For example, it has been proposed to dispose a grid for absorbing the scattered radiation between the object and the stimulable phosphor sheet. The grid comprises lead plates having a thickness of, for example, 1 mm or less and combined in a grid form or a multiple row form. When the grid is disposed as mentioned above, the scattered radiation advancing in random directions is absorbed by the lead plates. However, in the case of radiation image recording of the human body, even when such a grid is used, the amount of the scattered radiation is considerably large and is approximately equal to the amount of the main transmitted radiation, and therefore it is not possible to eliminate the scattered radiation completely with the grid.
It has also been proposed to position a first slit plate and a second slit plate respectively between the radiation source such as the X-ray tube and the object, and between the object and the stimulable phosphor sheet, to emit a radiation in a fan beam form, and to move the radiation source and the slit plates in synchronization with each other, thereby scanning the object with the radiation in the fan beam form. In this case, the radiation scattered by the object is intercepted by the second slit plate and does not impinge upon the stimulable phosphor sheet. However, with this method, since it takes approximately two to five seconds for radiation image recording and the object readily moves due to body motion or the like during this time, a motion artifact readily arises in the recorded image.
It has also been proposed to conduct signal processing for eliminating the adverse effect of the scattered radiation on the read-out image signal obtained by photoelectrically detecting the light emitted by the stimulable phosphor sheet, thereby restoring the contrast of the reproduced visible image. However, with this method, since the signal processing is conducted after digitizing the read-out image signal, the signal representing fine contrast of the radiation image is affected by quantization errors, and the fine contrast cannot be expressed on the reproduced visible image.